1. Field of the Invention
The present invention relates to a photosensitive resin composition useful to manufacture the dry film photoresist for circuit design of printed circuit board (hereinafter referred to as "PCB").
2. Description of the Prior Art
Usually, dry film photoresist is prepared by coating a photosensitive resin composition in admixture with organic solvent on a base film, followed by drying it and then laminating a protecting film on the dried photoresist film. In order to manufacture a PCB, such dry film photoresist is uniformly applied on an epoxy sheet overlaid with a copper film or coated with copper, with the aid of a laminator. For this purpose, the dry film photoresist is first peeled off the protecting film, then stuck to the copper overlay, together with the base film. A desired circuit (photo-tool) is allowed to adhere closely to the base film which is then exposed to uv. The parts illuminated are cured while the covered parts remain uncured. Following removal of the base film, application of a developing solution removes the uncured parts from the board with the cured parts remaining to form a circuit.
Significant properties necessary to dry film PCB include rapid developing speed, flexibility of the film to be exposed, and resistance to plating chemicals. Of the components consisting of photosensitive resin composition photo-polymerizable monomer has a great influence on such properties, so that careful consideration must be taken to select it.
It is well known that a high degree of cure allows the photosensitive resin to be more resistant to chemicals. The high degree of cure may be achieved by utilizing photo polymerizable monomers of low molecular weight the representative examples of which include ethylene glycol di-(meth)acrylate, diethylene glycol di-(meth)acrylate, pentaerythritol triacrylate, trimethylol propane tri-(meth)acrylate, etoxylated trimethylol propane tri-(meth)acrylate, propoxylated trimethylol propane tri-(meth)acrylate, and hydroxy di-(meth)acryloxypropane.
Where such low molecular weight monomers are used in large quantities, the reactive end group of the monomer increases in number, leading to large free volume in the photosensitive resin composition. As the free volume increases, the developing speed increases because the developing solution can easily penetrate into the photosensitive resin. However, the overcure attributed to the raise in the number of the reactive end group leads the film to brittleness and frangibility.
Alternatively, photo-polymerizable monomers containing phenyl group are used to provide good chemical resistance for the photo-polymerizable resin. Typical examples of phenyl group-containing photo-polymerizable monomer include compounds that contain bisphenol A intramolcularly, such as 2,2-bis4-(acryloxydiethoxy)phenyl!propane and 2,2-bis4-(methacryloxydiethoxy)phenylpropane. The abundance of such phenyl-containing monomers in photosensitive resin composition retards the penetration of weak alkaline aqueous solution, deleteriously affecting the developing speed.
In order to raise the developing speed, photo-polymerizable monomers carrying ethylene glycol unit can be added. As mentioned above, the use of low molecular weight monomers with few ethylene glycol units results in fragile film. On the other hand, photo polymerizable monomers with many ethylene glycol units enable the dry film photoresist to be dissolved in water, increasing the developing speed because the ethylene glycol is hydrophilic. In addition, the density of cure can be maintained at such levels as can induce high toughness after exposure to UV. However, the ethylene glycol leads to a significant decrease in plating chemical resistance. As will be described in greater detail later on, the plating chemical resistance can be improved by use of a combination of monomers carrying ethylene glycol and monomers carrying bisphenol A. Each monomer alone is found to be insufficient.
The flexibility of a dry film following exposure to light depends largely on the degree of cure and the structure of monomers used. For example, a large population of curable monomers in the composition causes frequent cure opportunities, resulting in poor flexibility. On the other hand, a small number of curable monomers provides a flexible film but inferior in plating chemical resistance. In manufacturing PCB, the flexibility of the film cured by being exposed to light largely determines its tentability, an ability to block up holes. The better the flexibility, the higher the tentability, resulting in an improvement in productivity.